Running Docker containers as non root

Running containers as root is a bad practice, but many Docker images available in the Docker Hub have the user set to root by default, so what can we do about it?

TL;DR Use -u 65534 -w /tmp -e _JAVA_OPTIONS=-Duser.home=/tmp for a typical Java image, plus any tool specific environment variable needed

Option 1 (build time):

Create a new derived image that creates a new user and changes the default to that one


FROM openjdk:8-jdk
RUN useradd --create-home -s /bin/bash user
WORKDIR /home/user
USER user

This is simple, but forces us to republish all these derived images, creating a maintenance nightmare.

Option 2 (runtime):

Use docker run -u option to choose what user to run the container as

docker run -ti --rm -u 1000 openjdk:8-jdk

This may work, but we can hit some issues, let’s see

$docker run -ti --rm -u 1000 openjdk:8-jdk git clone https://github.com/jenkinsci/docker
fatal: could not create work tree dir 'docker'.: Permission denied

Well, we obviously don’t have permissions to write to the default workdir, let’s fix it using -w and a dir that is writable, for instance /tmp

$ docker run -ti --rm -u 1000 -w /tmp openjdk:8-jdk git clone https://github.com/jenkinsci/docker
Cloning into 'docker'...
remote: Counting objects: 1498, done.
remote: Total 1498 (delta 0), reused 0 (delta 0), pack-reused 1498
Receiving objects: 100% (1498/1498), 287.46 KiB | 0 bytes/s, done.
Resolving deltas: 100% (772/772), done.
Checking connectivity... done.
fatal: unable to look up current user in the passwd file: no such user

 

Git does not like being run as an user that does not exist, so we need to pick one of the existing users

$ docker run -ti --rm -u 1000 -w /tmp openjdk:8-jdk cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
systemd-timesync:x:100:103:systemd Time Synchronization,,,:/run/systemd:/bin/false
systemd-network:x:101:104:systemd Network Management,,,:/run/systemd/netif:/bin/false
systemd-resolve:x:102:105:systemd Resolver,,,:/run/systemd/resolve:/bin/false
systemd-bus-proxy:x:103:106:systemd Bus Proxy,,,:/run/systemd:/bin/false
messagebus:x:104:108::/var/run/dbus:/bin/false

The user nobody:65534 could be a good candidate, and as it is present in the default debian and alpine Docker images it will be present also in most images in the hub.

$ docker run -ti --rm -u 65534 -w /tmp openjdk:8-jdk \
git clone https://github.com/jenkinsci/docker
Cloning into 'docker'...
remote: Counting objects: 1498, done.
remote: Total 1498 (delta 0), reused 0 (delta 0), pack-reused 1498
Receiving objects: 100% (1498/1498), 287.46 KiB | 0 bytes/s, done.
Resolving deltas: 100% (772/772), done.
Checking connectivity... done.

Ok, that worked! Now let’s try to run something else, like a maven build


$ docker run -ti --rm -u 65534 -w /tmp maven:3 \
bash -c "git clone https://github.com/jenkinsci/kubernetes-plugin.git && \
cd kubernetes-plugin && mvn package"
touch: cannot touch ‘/root/.m2/copy_reference_file.log’: Permission denied
Can not write to /root/.m2/copy_reference_file.log. Wrong volume permissions?

This means entering in the domain of each tool and checking how to configure it. The maven docker image instructs us to use MAVEN_CONFIG and pass -Duser.home otherwise we would get an error [ERROR] Could not create local repository at /nonexistent/.m2/repository -> [Help 1]

Here is the working solution


$ docker run -ti --rm -u 65534 -w /tmp -e MAVEN_CONFIG=/tmp maven:3 \
bash -c "git clone https://github.com/jenkinsci/kubernetes-plugin.git && \
cd kubernetes-plugin && mvn -Duser.home=/tmp package"

Can we generalize this a bit for other Java apps? yes! By using the env var _JAVA_OPTIONS we can pass the user.home property to any Java app.

$ docker run -ti --rm -u 65534 -w /tmp -e _JAVA_OPTIONS=-Duser.home=/tmp \
-e MAVEN_CONFIG=/tmp maven:3 \
bash -c "git clone https://github.com/jenkinsci/kubernetes-plugin.git && \
cd kubernetes-plugin && mvn package"